Digital pre-distortion (DPD) is a technique used to linearize a power amplifier in a transmitter to improve the efficiency of the power amplifier. A digital pre-distortion circuit inversely models the gain and phase characteristics of the power amplifier and, when combined with the amplifier, produces an overall system that is more linear and reduces distortion than would otherwise be caused by the power amplifier. An inverse distortion is introduced into the input of the amplifier, thereby reducing any non-linearity that the amplifier might otherwise exhibit.
PCT Patent Application No. PCT/US12/62186, filed Oct. 26, 2012, entitled “Processor Having Instruction Set with User-Defined Non-Linear Functions for Digital Pre-Distortion (DPD) and Other Non-Linear Applications,” discloses non-linear functions that include one or more parameters specified by a user, such as filter coefficient values or values from a look-up table. The disclosed DPD techniques are based on a generalized memory polynomial (GMP) model and implemented using look-up tables. Polynomial models, however, do not provide adequate performance for a number of common situations. For example, polynomial models do not capture functions that have discontinuities in them (such as a discontinuity of amplitude or derivative, or a higher order derivative).
While polynomial models have significantly improved the performance of software implementations of DPD and other non-linear applications, a need remains for improved techniques for modeling non-linear systems with discontinuities of amplitude, derivative or higher order derivatives.